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This article provides a review of the MIPS function and stack concepts, as well as character and string operations. It also introduces SPIM syscalls and demonstrates examples using syscalls for integers, characters, strings, and floating-point numbers.
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Review • Function • A consecutive piece of code doing a specific thing • To go to the function, use jal Function, which does two things: • goes to the code starting at the address associated with label Function, • stores the address of the instruction immediately following the jal instruction into $ra. • To return from the function, use jr $ra, which takes the code back to the instruction following the jal instruction. a a add $t0, $t1, $t0 a a sub $t2, $t1, $t0 a a jal Function a a sll $t0, $t0, 2 . . . Function: a a add $t0, $t1, $t0 a a sub $t2, $t1, $t0 a a jr $ra
Review • Stack • A piece of memory • Last in first out • Use $sp to keep track of the first used element on the stack
Character and String Operations • Characters are encoded as 0’s and 1’s using ASCII most commonly • American Standard Code for Information Interchange • Each character is represented using 8 bits (or a byte) • If stored within an integer, the 8 bit character portion will be located within the lowermost ordered bits; this allows you to optionally store characters within integers • MIPS provides instructions to move bytes • Load byte (lb) loads a byte to the rightmost 8 bits of a register • Store byte (sb) write the rightmost 8 bits of a register to memory week04-3.ppt
SPIM syscalls • Syscalls are operations defined within the assembler (not the processor) • The syscall operation is dependent on the value in $v0 • The parameters to the operation are in $a0-$a3 (except for floating point numbers and then it is $f12) • Read operations store the value back in $v0 (again except for floating point numbers and then it is $f0) week04-3.ppt
SPIM syscallsIntegers li $v0,1 # print an integer in $a0 li $a0,100 syscall li $v0,5 # read an integer into $v0 syscall
SPIM syscallsCharacters li $v0,11 # print a character in $a0 li $a0,’a’ syscall li $v0,12 # read a character into $v0 syscall
SPIM syscallsStrings li $v0,4 # print an ASCIIZ string at $a0 la $a0,msg_hello syscall Don’t worry about reading in strings
SPIM syscallsFloating Point li $v0,2 # print a single precision li $f12,5.5 # floating point number in $f12 syscall li $v0,3 # print a double precision li $f12,5.5 # floating point number in $f12 syscall li $v0,6 # read a single precision syscall # floating point number into $f0 li $v0,7 # read a double precision syscall # floating point number into $f0
SPIM syscallsOthers li $v0,10 #exit syscall
String Copy Procedure week04-3.ppt
.data msg_hello: .asciiz "Hello\n" msg_empty: .space 400 .text .globl main main: done: li $v0,4 la $a0,msg_hello syscall li $v0,4 la $a0,msg_empty syscall la $a0,msg_empty #dst la $a1,msg_hello #src jal strcpy li $v0,4 la $a0,msg_empty syscall li $v0,10 #exit syscall strcpy: lb $t0, 0($a1) sb $t0, 0($a0) addi $a0, $a0, 1 addi $a1, $a1, 1 bne $t0, $0, strcpy jr $ra
Stack • Key things to keep in mind: • Stack is a software concept – last in first out, that’s it. • In MIPS, you implement the stack by yourself by keeping $sp always pointing to the top element on the stack • Stack can be used in functions to save register values, and is the standard approach to save register values. But • You can also use stack for other purposes • This is not the only way to save register values.
.data msg: .asciiz "hello world" endl: .asciiz "\n" .text .globl main main: addi $sp,$sp,-1 sb $0,0($sp) la $t1, msg L0: lb $t0,0($t1) beq $t0,$0, L1 addi $sp,$sp,-1 sb $t0,0($sp) addi $t1,$t1,1 j L0 L1: la $t1,msg L2: lb $t0,0($sp) addi $sp,$sp,1 sb $t0,0($t1) beq $t0, $0, L3 addi $t1,$t1,1 j L2 L3: la $a0,msg li $v0,4 syscall la $a0,endl li $v0,4 syscall li $v0,10 #exit syscall
